Implantable ball and cup (socket) joints that are commercially available commonly consist of an implantable cup or socket that provides a generally hemispherical surface and that is made from a plastic such as ultra high molecular weight polyethylene, and a ball member that is made from a metal such as cobalt-chrome alloys. Usage of the implanted joint can result in significant wearing of the plastic cup portion, and this can in turn lead to such problems as wear particle-induced osteolysis which involves the development of large, soft tissue cysts where bone once existed. As a result of osteolysis, the bony support for both the cup and ball elements of a prosthesis may be weakened significantly, leading to loosening of these elements.
To reduce the amount of debris formed by wearing of a plastic surface, it would be desirable to make both the ball and cup articulating surfaces of a hard, wear-resistant substance such as a ceramic or a metal such as a cobalt-chrome alloy. The basic concept of utilizing metal-to-metal articulating surfaces for a prosthetic device is not new; reference is made to Walker, P. S., and B. L. Gold, The Tribology (Friction, Lubrication and Wear) of All-Metal Artificial Hip Joints, Wear 17:285-299, 1971, and to Notton, U.S. Pat. No. 4,263,681; Hintermann, U.S. Pat. No. 4,687,487; Sivash, U.S. Pat. No. 3,943,576; and Shersher, U.S. Pat. No. 3,859,669. The geometry of ball and cup prostheses also are discussed in Frey, U.S. Pat. No. 4,031,570; Muller, U.S. Pat. No. 4,784,662; and Fisher, PCT Publication No. WO 95/23566.
When the articulating surfaces of ball and cup joints are made of metal, however, wear continues to be a problem, it appears. There is an initial high rate of wear during the "wear in" period in which the ball or the cup or both are worn down to the point where they develop surfaces that fit with some snugness against one another, and as this occurs, the rate of wear gradually decreases. The small particles resulting particularly during the initial period of high wear are liberated into the synovial fluid that lubricates joint cavities and can be distributed into surrounding tissues; there is concern that this can lead to medical problems of the type encountered when polymeric surfaces are employed, as described above.
When the articulating surfaces of an all-metal ball and cup joint have worn down to the point where the wear rate has been reduced, commonly the wearing away process is found to have formed one or more ridges on the articulating surface of either the ball or the cup or both, the ridges delimiting the snug area or areas of contact between these surfaces. The ridges, while not interfering with normal articulating movement of the joint, none-the-less may accelerate wear during large range-of-motion movements and/or interfere with the entrainment of lubricating synovial fluid.